This invention relates to a method and apparatus for controlling a printer of the type whereby color indicia are printed by the selective energization of print heads which transfer a color dye from a dye medium to a record medium and, more particularly, to such a method and apparatus which compensates for non-linearities in the print characteristics of the dye medium and especially such non-linearities in the print characteristics of dyes of different colors.
Printing apparatus recently has been introduced for printing viewable images in response to video signals. Such apparatus serves to print a "hard copy" of a television picture. In a desirable embodiment thereof, a color image is printed in response to color video signals.
In the color printing apparatus of the type mentioned above, a pigmented medium is disposed between a record medium, such as paper, and a print head assembly formed of, for example, a plurality of individual print heads. When a particular print head is energized, the color indicium is printed on the record medium as a result of the transfer thereto of at least a portion of the pigment from the pigmented medium. The intensity, or darkness, of the printed indicium is a function of the intensity of the signal that is used to energize that print head.
One example of color printing apparatus of theaforedescribed type utilizes a thermal print head assembly, and is described in copending application Ser. No. 317,055, filed Nov. 2, 1981 now U.S. Pat. No. 4,389,935. In the color thermal printing apparatus disclosed therein, the intensity or darkness of the color indicium that is printed is a function of the duration of the energizing signal that is applied to the thermal print head. The longer the duration of that energizing signal, the darker or more intense the color indicium printed therefrom.
The pigmented medium which is used to print indicia of different colors preferably is in the form of a web containing a repetitive pattern of differently colored sublimable dyes. As described in copending application Ser. No. 384,284, filed June 2, 1982, the sublimable dye or other thermally transferable material vaporizes in response to heat applied thereto, transfers to the record medium and then condenses thereon. When forming a color image, a full image is printed from one colored section of the web in response to energizing signals derived from video signals of one color, and then the web is advanced to print a superimposed image from another colored section of the web in response to energizing signals derived from video signals of another color, and then the web is advanced once again to superimpose yet another colored image on the record medium. The resultant superimposed colored images are perceived as an integrated, full-color image.
It has been found that the print characteristics of the different colored sections of the dye medium used in the aforedescribed thermal print apparatus differ from each other. That is, the darkness, or intensity, of an indicium printed from one colored section in response to an energizing signal of given intensity or duration differs from the darkness or intensity of the image printed from another colored section in response to the same energizing signal. Also, the relationship between the darkness or intensity of indicia printed from a single colored section varies non-linearly with the intensity or duration of the energizing signals that are used to print those indicia. Such non-linearities in the print characteristics of the respective colored sections of the dye medium may be thought of as being analogous to so-called gamma deviations in the color image pick-up tubes used in color television cameras.